The scintillation detector module 1, as shown in FIG. 1, is widely used in positron emission tomography and single photon emission tomography. The scintillation detector module 1 comprises a scintillation crystals array 10 for converting gamma photons into scintillation photons. The crystal array 10 comprises a plurality of scintillation crystals 100 coupled with photomultiplier tubes (PMTs) 11. Each PMT has a photocathode to emit photo-electrons converted from scintillation photons by the scintillation crystals. The photo-electrons bombard the dynode to cause secondary emission and emit a plurality of secondary electrons. The secondary electrons are amplified after hitting the dynode. Finally, the anode collects all the emitted electrons to generate an output pulse signal. The intensity of the output signal depends on the number of photons received by the photocathode. The photons are processed by a readout electronic circuit to output gamma rays and crystal interaction location signals. If a uniform gamma-ray source continuously illuminates a detector module, the gamma rays and crystal interaction location signals can be shown as in FIG. 2A.
The identification of crystal interaction locations is important for identification of radiation source using the scintillation detector. In FIG. 2A, a crystal map with respect to a plane source detected by a detector module is shown. In the crystal map, each group of pixels represents a crystal response. The crystal map varies with crystal elements, photomultiplier tubes (PMTs) and electronic circuits and also changes with the voltage, gain and system reliability of photomultiplier tubes (PMTs). The groups of points corresponding to each crystal element are not arranged as regularly as the crystal elements in the crystal array. In order to determine the location where gamma rays interact with the imaging detector, a crystal look-up table is required to interpret the location where the gamma rays interact with the imaging detector. Therefore, the sensing signals correspond to respective crystal location in the crystal array for image reconstruction. To generate the look-up table, a uniform plane source is used to generate a crystal map, which is then divided and defined. By the use of conventional techniques such as mean shift, each group of pixels is denoted by a peak point so that a crystal location corresponding to each peak point can be defined.
Conventionally, the coordinate locations of a plurality of peak points are sorted along the X-axis direction or the Y-axis direction and then along the other. Such method is used for a crystal map with regular arrangement of crystals, but fails to apply a twisted crystal map. For example, FIG. 2B shows such method. In FIG. 2B, the coordinate locations are sorted along the Y-axis direction to determine twelve groups of points (in a box labeled 94), which are then sorted along the X-axis direction. In this case, the group of point 20 in the area 92 is neglected to cause errors in the area 93. Moreover, the peak points with respect to the crystals can also be found using the distance and the angle. But similarly, such method is not suitable for twisted crystal maps. The crystal locations corresponding to each peak point for twisted crystal maps are preferably acquired by human labors to find the area with respect to each crystal from the twisted crystal maps. Since most of the measurement results show twisted crystal maps, considerable manpower cost and time cost are required.